Pilot valve mechanisms, which may also be referred to as control valves, have long been utilized for the purpose of automatically responding to pressure variations in a flow system and inducing actuation or deactivation of mechanical devices in the event a dangerous or undesirable pressure level is sensed. As is typically the case, pilot valves or control valves may comprise a valve body having a pressure responsive valve element disposed therein for controlling communication between the intlet, outlet and vent port of the valve, whereby a mechanical device, such as a valve actuator, may be energized by pressurized pilot fluid passing through the valve and may be deenergized by blocking application of the pressurized pilot fluid and by allowing pilot fluid to be vented from the mechanical device through the pilot valve mechanism.
As is typically the case, a piston within the pilot valve is provided with a plurality of annular grooves having O-rings disposed therein, which O-rings slide across the inlet, outlet and port openings of the valve, depending upon the position of the shuttle valve. When high operating pressures are involved, travel of the O-rings across the ports subject small areas of the O-rings to substantial pressure differentials which causes the O-rings to be eroded quite rapidly by the valve structure at the ports, thus facilitating frequent repair and replacement. Moreover, it has been determined that excessively large ports create extremely rapid wear of the O-rings, while ports that are sufficiently small to prevent excessive O-ring wear typically retard the flow of pressurized medium through the pilot valve mechanism and thus prevent rapid actuation of the mechanical device that is being controlled. Depending upon the particular pressure range at which the pilot valve mechanisms are to function, certain optimum port sizes have been developed that allow sufficient flow for actuation at acceptable speeds even though the speeds are not necessarily optimum. Accordingly, it is desirable at times to provide pilot valve mechanisms for actuator systems having the capability of allowing rapid flow of pilot fluid for rapid actuation without the attendant difficulties that are often associated with the use of sliding O-rings of conventional pilot valve mechanisms.
Another problem typically associated with conventional pilot valve mechanisms is the tendency of O-rings to become completely displaced from their grooves as the result of excessive pressure differential or the tendency of such O-rings to become extruding from the groove to such extent that they become cut or excessively abraded as the O-ring slides passed various structural components of the pilot valve mechanism. It is therefore appropriated to provide a pilot valve mechanism having a facility for positively retaining O-rings in their proper position and for preventing such O-rings from becoming extruded from their grooves.
Where conventional O-rings are employed as dynamic sealing elements and are disposed in sliding engagement with a cylindrical wall defining a bore, such as is typically the case in most commercially available pilot valve mechanisms, the O-ring, after remaining stationary for a suitable period of time, will become adhered to the wall structure defining the bore to such extent that substantial pressure is necessary to break it loose and accomplish the desired actuation. Depending upon the characteristics under which the O-rings operate, it may require pressure in the order of 300 psi to break certain O-rings loose from the wall structure of the bore and to achieve linear actuation of the piston carrying the O-rings. Of course, when the O-ring breaks loose and movement occurs, the piston typically slams to the opposite position because of the pressure differential that is necessary to initiate piston operation. This usually results in erratic and undesirable operation of the pilot valve mechanism causing consequent erractic operation of the mechanical device with which the pilot valve is associated.
Sticking of O-rings is due largely to the fact that surface areas of piston bores have a certain degree of surface imperfection, depending upon the characteristics of the machining operation producing the bore. The material from which most O-rings are composed has a facility for extrusion into the surface imperfections and may have a characteristic of establishing a permanent or semi-permanent set after a certain period of time, thereby causing the O-ring to become mechanically adhered to the wall structure of the bore. It is desirable, therefore, to provide a sealing element having a material in sealing engagement with the bore, which material will be substantially frictionless, to allow relatively free movement of the piston in the bore and which material will not readily extrude into the surface imperfections of the piston bore and become adhered to such surface imperfections.
It is typical for compression springs to be employed in pilot valve mechanisms for imparting a force to a movable shuttle valve assembly that controls the flow of control fluid through the pilot valve mechanism. This is true primarily because the structure necessary for containing compression springs is much simpler and less expensive than other urging devices, such as tension springs, for example. Where control pressure ranges are fairly wide and exceptional repeatability is not a governing factor, pilot valve mechanisms incorporating compression spring devices for shuttle valve actuation are quite acceptable. They are less desirable when the opposite parameters are controlling.
When the coils or convolutions of typical compression springs overlap, it is obvious that the spring rate of the spring is substantially altered. When this occurs, a pilot valve mechanism will also have altered pressure responsive characteristics which may render the valve completely unacceptable for the service for which it is intended. Alteration of the pressure to which the pilot valve mechanisms will respond, may create an unsafe condition if the altered response pressure of the valve is excessively high or low. If the shuttle travel of the valve mechanism is great enough to create a condition were coil interference of the compression spring can occur, it may be undesirable to place such a valve mechanism in a critical environment.
Another undesirable pilot valve pressure responsive condition results when compression springs of pilot valves bend or buckle of such extent that the spring is allowed to rub against an internal surface of the valve mechanism or against a surface of a spring guide. When this occurs, the frictional rubbing engagement of the spring against another surface will substantially alter the spring rate of the spring and will interfere with normal spring function. The occurrence of spring rubbing will obviously modify the pressure range to which the pilot valve mechanism will respond and will frequently render such pilot valve mechanisms undesirable for use in environments where accurate pressure response is critical. Spring rubbing has the effect of broadening the range of pilot valve pressure response which renders compression spring type pilot valves undesirable if the high and low pressures to which the valve must respond are narrowly spaced. Rubbing of the spring will generally create sufficient frictional interference that pilot valves, so constructed, will not respond accurately to low pressures.
It has been considered desirable in the past to achieve piston movement by means of a bellows, because the bellows allows substantially friction-free movement of the piston and yet positively prevents actuating fluid from bypassing the piston and interfering with positive operation of the pilot valve mechanism. Bellows actuation of the piston of a pilot valve mechanism has not been readily acceptable in the past because substantial piston movement is typically required in order to allow sealing elements carried by the piston to move across the ports of the pilot valve and this large amount of movement causes the bellows to become weakened and to fall within a short period of time. It is appropriate, therefore, to provide means for allowing actuation of the piston of a pilot valve mechanism by means of a bellows and to facilitate long life of the bellows through use of a piston mechanism having limited movement.
Accordingly, it is a primary object of the present invention to provide a novel pilot valve mechanism suitable for both high and low pressure actuation, which pilot valve mechanism employs a piston and sealing arrangement allowing a large amount of flow to occur through the pilot valve mechanism with minimal pressure responsive movement of the piston and valve assembly thereof.
It is another important object of the present invention to provide a novel pilot valve mechanism employing sealing elements that promote effective substantially friction-free sealing, promote ease of valve and piston actuation and which do not tend to adhere to the wall structure of the valve mechanism and interfere with relatively free piston and valve movement.
Among the several objects of the present invention is noted the contemplation of a novel pilot valve mechanism developing a flow passage through the valve upon opening of a shuttle valve mechanism, which flow passage is of a dimension at least as great as the dimension of the inlet, outlet or vent ports of the valve mechanism in order to facilitate maximum fluid flow for rapid shut-in of the mechanical device with which the pilot valve mechanism is associated.
It is an even further object of the present invention to provide a novel pilot valve mechanism employing and urging means to oppose movement of the piston and valve mechanism of the pilot valve assembly and which cooperates with the piston and pilot valve mechanism to facilitate effective and accurate operation in narrow ranges of pressure differential operation of the valve mechanism and to achieve accurate high and low pressure operational repeatability.
It is also an important object of the present invention to provide a novel pilot valve mechanism incorporating a tension spring mechanism for imparting a pressure controlling force to a shuttle valve mechanism, which tension spring mechanism is not affected by friction interference and coil override and is accurately responsive a full range of fluid pressures.
It is also an important object of the present invention to provide a pilot valve mechanism that is of simple construction, is reliable in use and low in cost.
Other and further objects, advantages and features of the present invention will become apparent to one skilled in the art upon full consideration of the matter disclosed herein. The form of the invention, which will now be described in detail, illustrates the general principles of the invention, but it is to be understood that this detailed description is not to be taken as limiting the scope of the present invention.